Ok, so I'm the rare person who regularly uses both Spotify and YTM. I have used a variety of tools to transfer and sync playlists across the 2 services—Houdini and Soundiiz primarily. Source: about 1 year ago

Use https://soundiiz.com/ to migrate your playlists to other services. I paid a few bucks for one month of subscription and transferred a boatload of playlists (even ones that Spotify generated for me, like my previous Wrapped 'lists) over to YT Music. Source: 6 months ago

For those wondering what I used to convert the playlist, its TuneMyMusic. Theres a better alternative but it has more paid features. Soundiiz. Source: 6 months ago

Although you’ll want to use a service like https://soundiiz.com to transfer your playlist or songs if you added if you want to switch music service too. Source: 6 months ago

Or soundiiz.com can create that I think, but not in order (not sure at 100%). Source: 11 months ago

The good thing is, you'll still have all of your media sorted and organized on whatever hardware was hosting Plex, so you'll just need to install new software to host it. Make sure you're using something like Web Tools or Soundiiz to keep records of any playlists or collections and you'll be fine. Source: 11 months ago